For visibility, I'd rely on a bright jersey / jacket. Colored panniers start getting greased up / and or faded from the sun pretty quickly. I'd go with black. Roasting is good or bad depending upon humidity and what you're carrying. If your panniers breathe, then this will be enhanced in low humidity / high heat situations. Packaged non-perishable food will be fine. Perishible food will perish no matter what color your panniers are.